﻿using System;
using System.Collections.Generic;

namespace GB28181.Persistence
{

    internal static class TypeSystem {

        internal static Type GetElementType(Type seqType) {
            Type ienum = FindIEnumerable(seqType);
            if (ienum == null) return seqType;
            return ienum.GetGenericArguments()[0];
        }

        private static Type FindIEnumerable(Type seqType) {
            if (seqType == null || seqType == typeof(string)) {
                return null;
            }
            if (seqType.IsArray) {
                return typeof(IEnumerable<>).MakeGenericType(seqType.GetElementType());
            }
            if (seqType.IsGenericType) {
                foreach (Type arg in seqType.GetGenericArguments()) {
                    Type ienum = typeof(IEnumerable<>).MakeGenericType(arg);

                    if (ienum.IsAssignableFrom(seqType)) {
                        return ienum;
                    }
                }
            }

            Type[] ifaces = seqType.GetInterfaces();

            if (ifaces != null && ifaces.Length > 0) {
                foreach (Type iface in ifaces) {
                    Type ienum = FindIEnumerable(iface);
                    if (ienum != null) return ienum;
                }
            }

            if (seqType.BaseType != null && seqType.BaseType != typeof(object)) {
                return FindIEnumerable(seqType.BaseType);
            }
            return null;
        }
    }
}
